1. Field of the Invention
The present invention relates to semiconductor devices having a pin structure, such as diodes, transistors, thyristors, etc.
2. Description of the Background Art
FIGS. 15A, 15B show the structure of a conventional pin diode, where FIG. 15A shows its cross section and FIG. 15B shows its impurity concentration profile. The nxe2x88x92-type semiconductor substrate 601 mainly composed of silicon, for example, has a player 602 on one of its main surfaces (on the left in the drawing) and an n+ layer 603 on the other main surface (on the right in the drawing). For example, the player 602 contains boron or gallium as an impurity and the n+ layer 603 contains phosphorus, which are obtained by diffusing the impurity to a given depth by thermal treatment. The player 602 and the n+ layer 603 respectively have an anode electrode 604 and a cathode electrode 605 made of an electrically low resistant metal on the opposite sides to the nxe2x88x92-type semiconductor substrate 601.
The impurity in the nxe2x88x92-type semiconductor substrate 601 is almost uniformly distributed with a very small impurity concentration gradient. Since the player 602 and the n+ layer 603 are formed by applying impurity diffusion to the two main surfaces of the nxe2x88x92-type semiconductor substrate 601, the impurity concentration in each layer has an impurity concentration gradient decreasing toward the nxe2x88x92-type semiconductor substrate 601. For example, the n+ layer 603 has an impurity concentration gradient of about 4xc3x971018cmxe2x88x924. The impurity concentration gradient is given as a value obtained by dividing the difference between a first concentration that is 90% of the maximum value of the impurity concentration in the n+ layer 603 and a second concentration that is 50% of the maximum value by the distance from the position having the first concentration to the position having the second concentration.
Generally, when a reverse bias is applied to a diode having a pn junction in which a current is flowing in the forward direction by instantaneously switching an external circuit, a large reverse current transiently flows only in a certain period. This is caused by the accumulation of minority carriers in the diode, where the current does not immediately recover in the reverse direction even though it once reaches zero. This reverse current lasts until the minority carriers remaining as excess carriers in the vicinity of the junction decrease below a certain concentration and a depletion layer is established.
When a depletion layer is established, it starts supporting the reverse voltage, and the reverse voltage gradually increases and the reverse current gradually decreases as the depletion layer expands. Then the device voltage becomes steady equal to the voltage applied as the reverse bias and the reverse recovery operation is thus completed. The reverse current flowing in the reverse recovery operation decreases at a current decreasing rate that is determined by the reverse bias value and the inductance of the external circuit.
In the diode shown in FIG. 15, a center of carrier recombination is formed by proton irradiation etc. in the vicinity of the pn junction formed by the player 602 and the nxe2x88x92-type semiconductor substrate 601 so as to locally shorten the lifetime in the vicinity of the pn junction, thus providing a characteristic with lower forward voltage, smaller reverse recovery current, and high di/dt tolerance. The entirety of the nxe2x88x92-type semiconductor substrate 601 is subjected to diffusion of heavy metal, irradiation of electron beam etc. to set the carrier lifetime short.
However, when the reverse bias voltage is high, the applied voltage of the diode rapidly oscillates in the vicinity of completion of the reverse recovery operation, which generates such noise as may cause malfunction of the peripheral electric equipment. FIG. 16 is a graph showing the time variation in voltage VA and current IA during the reverse recovery operation of the diode shown in FIG. 15, where the time at which the forward bias is switched to the reverse bias by an external circuit is set as zero. As can be seen from the graph, the current of the diode becomes steady zero when about 8 xcexcs passed after the switching and a voltage oscillation having an amplitude xcex94V exceeding 2000 V occurs immediately after that.
It is supposed that such voltage oscillation is caused by resonance of an LCR series circuit formed by the diode and external circuit. The LCR series circuit is formed of a capacitance component C defined by the depletion layer and excess carriers of the diode as parameters, a resistance component R defined by the applied voltage and leakage current of the diode and the recombination current of the excess carriers as parameters, and the inductance component L of the external circuit.
The capacitance component C and the resistance component R of the diode change with time. Especially, the resistance component R rapidly changes when the excess carriers outside the depletion layer disappear. Then the resonance condition of the LCR circuit is reached and the voltage will oscillate as shown in FIG. 16. When the depletion layer reaches the n+ layer 603, the capacitance component C rapidly changes, which may trigger the voltage oscillation.
Such voltage oscillation can occur not only in diodes but also in GCT (gate controlled turn-off) thyristors with rapid switching rate when the voltage rises in turn-off operation. Such voltage oscillation is undesired because it causes such noise as may cause malfunction of the peripheral electric equipment.
For example, Japanese Patent Laying-open No.62-115880 discloses a technique for controlling the semiconductor layer and concentration to improve the waveform in reverse recovery operation.
A first aspect of the present invention is directed to a semiconductor device comprising a first semiconductor layer of a first conductivity type, a second semiconductor layer of a second conductivity type, and a semiconductor substrate which is interposed between the first semiconductor layer and the second semiconductor layer and has the second conductivity type with an impurity concentration lower than that in the second semiconductor layer. According to the invention, in the semiconductor device, the second semiconductor layer has an impurity concentration decreasing toward the semiconductor substrate, and the impurity concentration in the second semiconductor layer decreases from 90% of its maximum value to 50% thereof at an impurity concentration gradient equal to or less than 2xc3x971018cmxe2x88x924.
Preferably, according to a second aspect of the invention, in the semiconductor device, the maximum value of the impurity concentration in the second semiconductor layer is not more than 5xc3x971015cmxe2x88x923.
Preferably, according to a third aspect of the invention, the semiconductor device further comprises a third semiconductor layer of the second conductivity type formed so that the second semiconductor layer is sandwiched between the semiconductor substrate and the third semiconductor layer, and a metal electrode formed so that the third semiconductor layer is sandwiched between the second semiconductor layer and the metal electrode, wherein the third semiconductor layer has a surface impurity concentration of not less than 5xc3x971017cmxe2x88x923 on the side of the metal electrode.
A fourth aspect of the invention is directed to a semiconductor device comprising a first semiconductor layer of a first conductivity type, a second semiconductor layer of a second conductivity type, and a semiconductor substrate which is interposed between the first semiconductor layer and the second semiconductor layer and has the second conductivity type with an impurity concentration lower than that in the second semiconductor layer. According to the semiconductor device of the fourth aspect, in a region in which a depletion layer extends when a reverse bias is applied to pn junction between the first semiconductor layer and the semiconductor substrate, the ratio of the amount of impurity in the semiconductor substrate to the amount of impurity in the first semiconductor layer is not more than 2/3.
Preferably, according to a fifth aspect of the invention, in the semiconductor device, in the region in which the depletion layer extends, the ratio of the amount of impurity in the second semiconductor layer to the amount of impurity in the first semiconductor layer is not less than 1.
Preferably, according to a sixth aspect of the invention, in the semiconductor device, in the region in which the depletion layer extends, the ratio of the amount of impurity in the second semiconductor layer to the amount of impurity in the first semiconductor layer is not less than 3/2.
According to the semiconductor device of the first aspect of the invention, the second semiconductor layer has a gentle impurity concentration gradient, which suppresses rapid expansion of the depletion layer in reverse recovery operation of a diode having the pn junction formed by the first semiconductor layer and the semiconductor substrate and in turn-off operation of a transistor or a GCT thyristor having the pn junction. Accordingly, it is possible to suppress unwanted voltage variation in the above-mentioned operations, or generation of noise.
According to the semiconductor device of the second aspect of the invention, the impurity concentration gradient mentioned in the first aspect can be obtained easily.
According to the semiconductor device of the third aspect, while the maximum value of the impurity concentration in the second semiconductor layer can be reduced to readily obtain a favorable impurity concentration gradient, the presence of the third semiconductor layer having high impurity concentration provides good ohmic contact between the metal electrode and the second semiconductor layer.
According to the semiconductor device of the fourth aspect, the semiconductor device can be formed thinner to realize smaller on-state voltage.
According to the semiconductor device of the fifth aspect, it is possible to reduce the leakage current.
According to the semiconductor device of the sixth aspect, the leakage current can be 10 mA or smaller.
The present invention has been made to solve the problems described above, and an object of the present invention is to provide a semiconductor device in which oscillation of the applied voltage is small in the reverse recovery operation or turn-off operation.